Fluorescence methods were used to study certain problems of concern in biomedical sciences. The areas investigated included: 1) the effect of Vitamin A analogs on the fluidity of cell membrane lipids in relationship to the toxicity of the analogs. 2) the binding of metal ions to tryptophan peptides, as determined by fluorescence quenching. 3) the functional integrity of phospholipases. The changes in membrane structure were followed by a fluorescent dye leakage method. 4) the effects of salt and phospholipids on the activity of a number of enzymes such as alcohol dehydrogenases and glutamate dehydrogenase.